Project Summary/Abstract Delayed childbearing is increasingly common in the industrialized world. In 2013, 15% of babies in the United States were born to women older than 35 years and 3% to women older than 40 years, a dramatic increase from only 8% of births to women older than 35 and 1% to women older than 40 years in 1988. Pregnancies to older women are at increased risk for adverse outcomes compared to pregnancies among younger women. Unfortunately most previous studies in this area have dichotomized maternal age, giving stakeholders the mistaken impression that a threshold between `older' and `younger' ages for childbearing exists at age 35 or 40. Moreover, women who have a first live birth at an older age have shorter inter-pregnancy intervals compared to women at younger ages. Short inter-pregnancy intervals (<18 months) are associated with increased risks of many important adverse pregnancy outcomes. When planning the next pregnancy, older women have to balance the potential risks associated with short intervals against their increasing age with longer intervals. The optimal inter-pregnancy interval for women who delayed their first pregnancy has not been identified. For many women, age at pregnancy is a modifiable risk factor for which preconceptional counseling tools are needed. For those women who have already delayed childbearing, pregnancy spacing for subsequent pregnancies is a modifiable risk factor for which evidence-based recommendations are needed. As delaying childbearing has great public health implications, it is critical for the evidence to be presented in informative and clearly interpretable formats for public health messaging at a population level. This study will address shortcomings in the literature regarding the relationship between age at pregnancy and adverse pregnancy outcomes and will identify the optimal inter-pregnancy interval for parturients older than 30. The following three aims will be achieved: i) Estimate the absolute risk of 14 pregnancy outcomes according to 1-year increments of maternal age at pregnancy to identify the dose-response relationship; ii) Use a causal framework to estimate the proportion of the effect of age at pregnancy on outcomes that would be eliminated by preventing complications of pregnancy among older mothers; iii) Identify the optimal inter-pregnancy interval for women who had a first birth older than 30. This study will be conducted using birth registry data from British Columbia (Canada) and the United States. These are ideal data sources for this study, with sample sizes that enable examination of rare outcomes and a granular analytic approach for maternal age. Pregnancy outcomes have large effects on the health of a population. As more women delay childbearing, the relationship between age at pregnancy and adverse pregnancy outcomes will have increasingly greater effects on the population's health. It is thus critical that risk factors for adverse pregnancy outcomes are well understood in the academic literature and effectively communicated to stakeholders.